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Note 8 - Stockholders' Equity - Warrants (Details) - $ / shares
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Outstanding, number of shares (in shares) 1,592,775  
Outstanding, weighted average exercise price (in dollars per share) $ 2.55  
Outstanding, weighted average remaining life (Year) 2 years 219 days 175 days
Issued, number of shares (in shares) 1,749,171  
Issued, weighted average exercise price (in dollars per share) $ 0.50  
Issued, weighted average remaining life (Year) 4 years 219 days  
Exercised, number of shares (in shares)  
Exercised, weighted average exercise price (in dollars per share)  
Expired or cancelled, number of shares (in shares)  
Expired or cancelled, weighted average exercise price (in dollars per share)  
Outstanding , number of shares (in shares) 3,341,946  
Outstanding , weighted average exercise price (in dollars per share) $ 1.48